Objectives
Anaplastic thyroid carcinoma (ATC) is a rare malignant tumor of the thyroid gland with an extremely poor prognosis. This study aimed to review ATC cases with long-term complete response (CR) and evaluate the factors contributing to a favorable prognosis.

Methods

We reviewed the medical records of six women with ATC who visited our institution. Four patients had stage IVB, and two patients had stage IVC with pulmonary metastases.

Results

Of the four cases with stage IVB, two patients maintained long-term CR. Details are as follows:

Case 1: A 60-year-old Japanese woman was referred to our department for evaluation of right supraclavicular fossa swelling. Pathological examination of an unresectable lesion in the supraclavicular fossa revealed a high-grade malignant tumor of unknown histological differentiation. Based on the cytological and imaging findings, lenvatinib was administered for ATC. The tumor significantly shrank within 2 weeks, and the patient has maintained CR for over 4 years.

Case 2: A 70-year-old Japanese woman presented with swelling in the left anterior neck. Hemithyroidectomy was performed, and histopathological findings revealed squamous cell differentiation around the follicular tumor, leading to a diagnosis of ATC. After confirming no distant metastases or other primary sites, the patient underwent a left neck dissection and additional CRT. Additional surgery was performed for lung metastasis of follicular carcinoma. The patient has maintained CR for 5 years.


Conclusions

We reported two cases of ATC with long-term CR. Case 1 was a malignant tumor arising in the supraclavicular fossa, which was challenging to histologically prove as primary thyroid cancer. Case 2 was diagnosed as ATC with squamous cell differentiation in follicular carcinoma. According to the 2022 World Health Organization classification, this condition is classified as ATC, not squamous cell carcinoma. We speculate that the atypical nature of these cases may contribute to their long-term CR.